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The cerebellum constitutes only 10% of the total volume of the brain but contains more than one-half of its neurons. The structure comprises a series of highly regular, repeating units, each of which contains the same basic microcircuit. Different regions of the cerebellum receive projections from different parts of the brain and spinal cord and project to different motor systems. Nonetheless, the similarity of the architecture and physiology in all regions of the cerebellum implies that different regions of the cerebellum perform similar computational operations on different inputs.

The symptoms of cerebellar damage in humans and experimental animals give the clear impression that the cerebellum participates in the control of movement. Thus we describe these symptoms because knowledge of them, in addition to being critical for the clinician, constrains conjecture about the exact role of the cerebellum in controlling behavior. The goal of cerebellar research is to understand how the connections and physiology of cerebellar neurons define the function of the cerebellum. Thus a major part of this chapter covers the fundamentals of cerebellar physiology and anatomy.

Finally, there is a relationship between cerebellar operation and more theoretical concepts of "internal models" in motor control (see Chapter 33). A fundamental precept of modern cerebellar research is that these internal representations of the external world are implemented in the cerebellum. The cerebellum could adjust motor performance by using its learning capabilities to alter the internal models to match any changes in the motor effectors of the external world. Thus at the conclusion of this chapter we discuss cerebellar learning and its possible relationship to internal models.